1. Field of the Invention
This invention pertains generally to in situ formation of reactive barriers in soil, and more specifically the invention relates to in situ formation of apatite barriers for containment of soil contaminants including radionuclides, such as actinides, and heavy metals.
2. Description of the Related Art
Apatite minerals (having, generally, the chemical formula Ca5 (PO4)3 X where X is a halide or hydroxyl) including hydroxyapatite are phosphate mineral chemically and morphologically similar to natural bone. Consequently, hydroxyapatite, for example, is commonly used in medical and dental applications as a synthetic substitute for bone. A number of studies over the past several decades have shown that radionuclides and heavy metals bind onto the surface of apatites in the fashion that they are almost totally immobilized. (Gauglitz, R, M. Holterdorf, W. Frank, and G. Marx, xe2x80x9cImmobilization of Actinides by Hydroxyapatite.xe2x80x9d, Mat Res. Symp. Proc. Vol 257 pp. 567-573 (1992); Jeanjean, J., J. C. Rouchaud, L. Tran, and M. Fedoroff, xe2x80x9cSorption of Uranium and Other Heavy Metals on Hydroxyapatite.xe2x80x9d, Radioanal. Nucl. Chem. Letters, vol. 201 (6) pp. 529-539 (1995); Arey, J. S., J. C. Seaman, and P. M. Bertsch, xe2x80x9clmmobilization of Uranium in Contaminated Sediments by Hydroxyapatite Additionxe2x80x9d, Environ. Sci. Technol. Vol. 33 pp. 337-342 (1999)). Consequently, the use of apatites in connection with immobilizing and containing radionuclides around contaminated sites and leaking storage containers has been the subject of active research.
Leakage of radioactive materials from storage tanks and subsequent migration of the radionuclides away from the containment area continues to be a significant unresolved problem at a number of government and private storage sites. Various techniques are used to try to isolate leaking storage containers and contaminated soil to prevent movement of contaminants into uncontaminated soil and especially groundwater.
One approach is simply to attempt to dig up and remove contaminated soil. This, however, is costly, and disturbance of contaminated soil carries the risk that some contaminants will be missed or released and left to migrate further. Excavation also has a negative effect on soil stability. Excessive digging and excavation around waste tanks, for example, has the potential to aggravate waste transport by shaking loose heavily corroded containment drums and already contaminated soil.
Another approach is to establish barriers in the soil of a contaminated site in order to prevent migration of contaminants beyond the barriers. Barriers of this sort that are in use at various sites around the United States and abroad include vertical sleeves placed in trenches surrounding a site. They also include walls formed through the injection of highly pressurized grout in holes drilled in the soil. Emplacement of such barriers typically requires greatly disturbing the soil and often there is no convenient way to create a xe2x80x9cfloorxe2x80x9d or continuous barrier beneath the leaking tank or contaminated region. Consequently, the sequestration of the contaminants is incomplete and contaminants continue to migrate downward and perhaps outward, thereafter. For areas under waste tanks, waste trenches and certain geological formations, forming a continuous barrier is difficult and sometimes impossible.
Various efforts have been made to create reactive barriers which, in addition to forming a physical obstruction to the movement of contaminant substances, actively attract and chemically bind contaminants. Using technology currently available, these chemically active materials can be combined with other components to form slurries that harden in the ground forming reactive barriers. Jet injection processes, for example, are known and used wherein machines pump slurries in holes drilled around the perimeter of a leaking vessel or contaminated site. Additionally, trenches can be dug and backfilled using chemically sorbent materials. Each of these techniques, however, carries the disadvantages previously mentioned relating to significant disturbance of the soil and difficulty in fully surrounding (or encapsulating) a leaking waste tank or region of contaminated soil.
It has been previously shown that mixing calcium chloride and sodium phosphate under basic conditions will yield brushite (CaHPO4.H2O). Then, the brushite can, over time, slowly change into hydroxyapatite through the mechanism of hydrolysis. (Momma, H and T. Kamiya, xe2x80x9cPreparation of Hydroxyapatite by the Hydrolysis of Brushite.xe2x80x9d, J. Mat. Sci. vol. 22 pp. 4247-4250 (1987); Boskey, A. L. and A. S. Posner xe2x80x9cFormation of Hydroxyapatite at Low Supersaturation.xe2x80x9d, J. Physical Chem., vol. 80(1) pp. 40-45 (1976)) Rapid, in situ formation of apatite in soil, as it is accomplished through the present invention, has not previously been demonstrated.
The need remains, therefore, for reactive barriers wherein apatite is synthesized rapidly in situ, in a way that does not disturb soil in the fashion that prior art methods tend to do, and that provides complete or near-complete containment and immobilization of contaminants present in a contaminated region of soil.
Therefore, it as an object of the present invention to provide reactive barriers capable of binding and inhibiting migration of soil contaminants including actinides and heavy metals.
It is another object of the invention to provide a method of in situ formation of reactive barriers in soil wherein those barriers include phosphate.
It is another object of the invention to provide barriers, formed in situ, wherein apatite is formed rapidly upon mixing of chemical reactants in soil.
It is yet another object of the invention to utilize capillary action of soil and/or movement of groundwater, and/or soil washing to help mix injected water-soluble reagents to react and chemically form a reactive barrier in the soil, itself.
An advantage of the present invention is that by adequately saturating soil with necessary chemical reagents, the barrier formed using the methods described here will be substantially continuous. Accordingly, a leaking container which is buried, for example, can be substantially encapsulated with apatite.
Another advantage of the present invention is that the processes of the invention can accommodate using low-pressure injection techniques to form the reactive barrier, where minimal disturbance of the soil is needed or desired.
Another advantage of the present invention is that the chemical processes involved are suited also to high-pressure injection of reagents, for circumstances wherein rapid injection is desired or required.
Another advantage of the present invention is that it is economical in comparison with traditional digging and excavation decontamination methods, and no backfilling is required.
These and other objects and advantages of the present invention are fulfilled and satisfied by the claimed invention which includes barriers and methods for making barriers in situ in soil. These barriers, which include reactive phosphate barriers such as apatite, are formed by injecting dilute reagents in soil under conditions of controlled temperature and pH where the reagents migrate through the soil and mix as a result of natural percolation. This mixing is mediated, for example, by capillary action, movement of groundwater, gravity and, where applied, injection pressure. The principles of the invention are suited to allow complete or near complete encapsulation of contaminant sources in soil, as well as to formation of walls or other barrier configurations.
Additional advantages and novel features will become apparent to those skilled in the art upon examination of the following description or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.